1. Field of the Invention
This invention relates to: a method for adjusting the timings at which laser light sources included in an image formation apparatus emit lights; a laser scanning apparatus; and an image formation apparatus including such a laser scanning apparatus.
2. Description of the Related Art
In recent years multi-beam laser scanning apparatus in which writing is performed by emitting a plurality of beams from a semiconductor laser array having a plurality of laser light sources and simultaneously scanning a plurality of scanning lines are used in image formation apparatus, such as printers and copying machines, in order to realize high-speed processing and high resolution.
If a laser scanning apparatus having two laser light sources is used, a defective image on which lines like scratches have appeared may be printed out due to the interference of light beams emitted from these laser light sources. A method in which the two laser light sources (first laser light source and second laser light source) are placed so that they will be inclined at an angle to a sub-scanning direction is known as a means of reducing such interference. In this case, as shown in FIG. 1, there is a deviation in a main scanning direction between beam spots of the first laser light source and the second laser light source. Accordingly, the timing at which these laser light sources emit light must be controlled to correct this deviation in the main scanning direction. Such a deviation in the main scanning direction may also occur when the laser light sources are in an inclined position due to an angular error in mounting. It is assumed that the mounting angle characteristic of the laser light sources is 1°=4.7 μm. To shift an image by one pixel, an angle of 9°(=42.21 μm) is required under 600 dpi.
A technique for detecting a deviation in the main scanning direction between beam spots of laser light sources and adjusting the deviation by reading an image which is formed on a photosensitive drum or which is printed out with a sensor, is disclosed in JP-Tokukaihei-10-221618A (hereinafter referred to as “patent document 1”) as such a laser scanning apparatus (multi-beam apparatus) having a plurality of laser light sources.
With the method disclosed in patent document 1, however, hardware, such as an image read sensor, is required. As a result, manufacturing costs rise. In addition, an image is formed and is then read. It takes time to perform a detection process, resulting in low processing efficiency.